1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device, and more particularly to a method of manufacturing a semiconductor device in which a resistance layer is formed of polysilicon.
2. Description of the Related Art
Many resistance layers are formed in a semiconductor device, and a polysilicon layer is used as the resistance layer for high integration and high density. As shown in FIG. 1, a polysilicon film 103 is formed on the insulating films 101 and 102 formed on a silicon substrate 100 to have a desired pattern. An interlayer insulating film 106 is formed on the polysilicon layer 103. Contact holes are formed in the interlayer insulating film 106 so that wiring films 107 are connected to the polysilicon layer 103. Thus, the resistance film 103 of the polysilicon is formed.
It is conventionally known that the polysilicon resistance films formed thus have a variance in resistance. A method of suppressing the resistance variance is described in Japanese Laid Open Patent Application (JP-A-Heisei 9-232521).
Also, it is known that the resistance values of the polysilicon films have a variance when hydrogen atoms are diffused into the polysilicon resistance film. To prevent the resistance variance due to the diffusion of hydrogen atoms, a technique is known in which a silicon nitride film is formed on the surface of the polysilicon resistance film.
However, impurity ions which have been implanted into the polysilicon resistance film to decrease a contact resistance are diffused into a resistance region of the polysilicon resistance film in a thermal process for the forming of the silicon nitride film. As a result, the resistance variance occurs. A method of preventing such a resistance variance is known in Japanese Laid Open Patent Application (JP-A-Heisei 9-121024).
The resistance variance of the polysilicon resistance film is also based on a variance of contact resistance. That is, as shown in FIG. 1, when the contact hole is formed by a plasma etching method, an opening portion of the polysilicon resistance film is cut because of over-etching, so that the contact resistance variance occurs.
Therefore, it is desired that the damage of the polysilicon resistance film because of the over-etching in the case of forming the contact hole is restrained so that the variance in contact resistance of the polysilicon film is retrained.